Compensation for frequency adjustment in mobile communication-positioning device with shared oscillator

ABSTRACT

In a mobile communication device, a method for compensating for a frequency adjustment in an oscillator shared between a communication circuit and a positioning signal receiver is provided. In one embodiment, the method begins to receive and store a positioning signal at a first time point. When, at a second time point, the operating frequency of the shared oscillator is adjusted, the frequency adjustment is recorded. After the positioning signal is completely received and stored, the processing of the positioning signal takes into consideration the frequency adjustment. In that embodiment, the processing hypothesizes a frequency shift in the received positioning signal. According to another aspect of the present invention, a method for determining the operating frequency of an oscillator detects a beginning time point of a reference signal received by the mobile communication device and enables a counter to count in step with a clock signal derived from the oscillator. When an ending time point of the reference signal is received by the mobile communication device, the count is stopped, and the frequency of the oscillator is determined based on the count in the counter and an expected time that elapsed between the beginning time point and the ending time point.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a mobile communication devicewith a positioning capability. In particular, the present inventionrelates to a mobile communication device (e.g., a cellular telephone)that is also capable of receiving a global positioning system (GPS)signal, and which shares an oscillator between the communication andpositioning functions.

[0003] 2. Description of the Related Art

[0004] The utility of a mobile communication device (e.g., a cellulartelephone) is enhanced if it is provided the additional capability ofreceiving and processing global positioning system (GPS) signals thatcan be used to determine the position of the mobile communicationdevice.

[0005] To provide for both positioning and communication functions, itis possible to share a local oscillator between the receiver andtransmitter of the communication circuit and the GPS signal receiver.While sharing a local oscillator can reduce the cost and bulkiness ofsuch a mobile communication device, there are some practical problems tobe overcome to achieve high performance. For example, in cellularcommunications, when a mobile communication device leaves the servicearea of a base station and enters into the service area of another basestation, a “hand-off” procedure takes place in which the mobilecommunication device tunes into the operating frequency or channel ofthe new base station. During the hand-off, it is often necessary toadjust the offset (i.e., deviation from the base station's “nominalcenter frequency”), as each base station may have a different offset. Indegraded signal conditions, continuous tracking of a carrier may alsorequire an offset frequency adjustment. However, if such an adjustmentis made during the acquisition of a GPS signal, both the mixingfrequency and the sampling frequency of the GPS receiver—used indown-converting and digitizing the GPS signal, respectively—areaffected. The received signal may yield an erroneous result, or even afailure to detect the GPS signal. In fact, in one system, a 0.05parts-per-million (ppm) adjustment has the effect of a 79 Hz shift inthe carrier frequency in the received GPS signal.

[0006] One approach avoids the corruption of the GPS signal by lockingthe communication circuit out from accessing the oscillator forfrequency adjustment so long as a GPS signal acquisition is in process.However, such an approach is undesirable because it prevents the mobilecommunication device from establishing contact with one or more basestations while a GPS signal is being acquired, which may lead totemporary loss of communication service. Also, such an approachcomplicates the control software in the mobile communication device,thereby deterring manufacturers from incorporating positioningcapability in their mobile communication devices.

[0007] In GPS signal detection, one source of uncertainty in the carriermodulation frequency in the received signal is the “clock Doppler,”which results from the unknown syntony between the clock on the signalsource (e.g., a GPS satellite) and the receiver's own clock. Preciseknowledge of the local oscillator's frequency can reduce the frequencysearch space (“Doppler range”) for the GPS signal. At any given time,the actual frequency of a local oscillator depends on a number ofvariables, such as manufacturing variations, temperature, aging andoperating voltage. Oscillators used in signal sources (e.g., GPSsatellites) are typically well-characterized and are tuned to thespecified frequency with high accuracy. Because of their cost, highpower requirements, and bulkiness, however, such oscillators areunsuited for use in a mobile communication device. To more accuratelydetermine the operating frequency of a local oscillator, the prior arttypically requires a more costly oscillator then conventionally found ina mobile communication device. Others require a complex calibrationprocedure to tune the oscillator to a precision carrier frequency. Thelatter approach is disclosed, for example, in U.S. Pat. No. 5,874,914 toKrasner, entitled “GPS Receiver utilizing a Communication Link.” Neitherapproach is satisfactory from a cost and performance standpoint.

SUMMARY

[0008] According to one embodiment of the present invention, provided ina mobile communication device, is a method for compensating for afrequency adjustment in an oscillator shared between a communicationcircuit and a positioning signal receiver. In one embodiment, the methodincludes (a) at a first point in time, beginning receiving and storinginto a storage device the positioning signal; (b) at a second timepoint, adjusting a frequency of the oscillator by a given amount; (c)recording the frequency adjustment; (d) at a third time point,completing receiving and storing of the positioning signal from thepositioning signal receiver; and (e) processing the positioning signal,taking into consideration the frequency adjustment. In oneimplementation, the second time point is recorded as the time at whichthe frequency adjustment of the oscillator is made. Having the knowledgeof the time at which the frequency adjustment is made, the processingsearches for a frequency shift in the received positioning signalbetween the second time and the third time. In another implementation,the amount by which the frequency of the oscillator is adjusted isrecorded, and the processing searches for a time point at which thefrequency adjustment of the oscillator is made. In one implementation,the processing integrates a correlation function.

[0009] The present invention is applicable to GPS processing usingaiding data, such as satellite ephemeris data. The present invention isparticularly applicable to cellular communication in which an oscillatoradjustment may be made when the mobile receiver moves between serviceareas of base stations.

[0010] Thus, accurate processing of the positioning data is ascertainedwithout preventing the communication circuit from accessing the sharedoscillator while positioning data is being acquired.

[0011] According to another aspect of the present invention, a mobilecommunication device determines an operating frequency of an oscillatorbased on a reference signal from a reliable time base. In oneembodiment, a beginning time point of the reference signal is receivedby the mobile communication device. When the beginning time point of thereference signal is detected, a counter is enabled to count a number ofcycles in a clock signal derived from the oscillator. The ending timepoint of the reference signal is then detected. Upon detecting theending time point of the reference signal, the counter is stopped toprevent the counter from further counting. Finally, the frequency of theoscillator is determined based on the count in the counter and anexpected time that elapsed between the beginning time point and theending time point.

[0012] The present invention can use reference signals having a knownduration in time, or having recurring events in the reference signalthat recurs at a fixed frequency. In some implementation, the frequencyof the oscillator so derived can be further adjusted, taking intoaccount the processing times in the mobile communication device fordetecting the beginning time point and the ending time point.

[0013] Using the method of the present invention, the operatingfrequency of a local oscillator can be determined to the accuracy of theoscillator of the base station oscillator, without incurring the expenseor inconvenient bulkiness of the more costly, higher precisionoscillator typically found in base stations or less mobile equipment. Ina GPS signal receiver, by removing the uncertainty in oscillatorfrequency, the Doppler range over which the positioning signal receiversoftware searches can be further limited.

[0014] The present invention is better understood upon consideration ofthe detailed description below and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a block diagram of mobile communication device 100 towhich a method of the present invention is applicable.

[0016]FIG. 2 shows a flow chart of method 200 that compensates for thefrequency adjustment in shared local oscillator 108, in accordance withone embodiment of the present invention.

[0017]FIG. 3 shows a block diagram of mobile communication device 100 towhich a method of the present invention is also applicable.

[0018]FIG. 4 illustrates method 400 for measuring the operatingfrequency of shared local oscillator 103, in accordance with oneembodiment of the present invention.

[0019] To facilitate comparison between figures and to simplify thedetailed description below, like reference numerals are used for likeelements in the figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention provides a method that compensates thefrequency adjustment effects in the positioning signal detectionprocess. According to another aspect of the present invention, a methodfor accurately determining the frequency of a local oscillator isprovided.

[0021]FIG. 1 shows a block diagram of mobile communication device 100 towhich a method of the present invention is applicable. Mobilecommunication device 100 can be, for example, a cellular telephonehandset. As shown in FIG. 1, mobile communication device 100 includescommunication receiver 101, communication transmitter 102, positioningsignal receiver 103, analog baseband circuit 106, digital basebandcircuit 107, shared local oscillator 108 and synthesizer 109. In mobilecommunication device 100, shared local oscillator 108 is the frequencysource for communication receiver 101, communication transmitter 102,and positioning signal receiver 103. Shared local oscillator 108 can beimplemented, for example, by a voltage-controlled oscillator. Antenna104 serves both communication receiver 101 and communication receiver102, and antenna 105 serves positioning signal receiver 103.

[0022] A communication signal coupled by antenna 104 into communicationreceiver 101 is band-pass filtered, amplified and then down-converted bymixing with a signal from synthesizer 109 to a baseband signal. (Thesignal from synthesizer 109 has the expected carrier modulationfrequency.) The baseband signal so obtained is low-pass filtered andsampled for digital processing in digital baseband circuit 107. Acommunication signal to be transmitted is provided as a digital signalfrom digital baseband circuit 107. The digital signal is converted intoanalog form, filtered and modulated by mixing with a carrier frequencyprovided by synthesizer 109. The modulated signal is amplified andtransmitted through antenna 104. The positioning signal received atpositioning signal receiver 103 is processed in substantially the samemanner as described for the communication signal, except that theexpected modulation carrier frequency, rather than generated by asynthesizer (e.g., synthesizer 109), is provided by a PLL, whichmultiplies the frequency of shared local oscillator 108 by a factor of100 or more.

[0023] Analog baseband circuit 106's functions include enablingcommunication transmitter 102 to transmit a communication signal,providing a frequency adjustment to shared local oscillator 108, andchanging the frequency in synthesizer 109. As mobile communicationdevice 100 switches between base stations, analog baseband circuit 106directs synthesizer 109 to switch between the channels of the basestations. As explained above, switching between base stations ortracking a carrier signal during degraded signal conditions maynecessitate a frequency adjustment to shared local oscillator 108. Inaddition, as shown in FIG. 1, analog baseband circuit 106 may include acodec and interfaces to a microphone and a speaker for processing voicecommunication. The codec quantizes a voice signal from the microphoneinto digital samples to be processed by digital baseband circuit 107 andreconstructs an analog audio or voice signal from digital samplesprovided by digital baseband circuit 107. The analog audio signal isreplayed at the speaker.

[0024] Digital baseband circuit 107 includes receiver interface (RXIF)111 and transmitter interface (TXIF) 112 to communication receiver 101and communication transmitter 102, respectively. The output signal ofshared local oscillator 108 provides a reference signal for clockgeneration circuit 113 to provide the internal clock signals distributedwithin digital baseband circuit 107 and analog baseband circuit 106(e.g., to drive sampling of a voice codec). The internal clock signalsgenerate from timing generation circuit 114 timing strobes used bothinternally in digital baseband circuit 107 and analog baseband circuit106. Various serial communication and input/output (I/O) ports 115 areprovided in digital baseband circuit 107 for communication withperipheral devices, positioning signal receiver 103 and analog basebandcircuit 106.

[0025] Digital baseband circuit 107, which can be implemented as anapplication specific integrated circuit (ASIC), includes centralprocessing unit (CPU) subsystem 131, which performs and controls thecommunication functions of mobile communication device 100. Suchcommunication functions include executing the communication protocolstack, peripheral hardware control, man-machine interface (e.g., keypadand graphical user interfaces), and any application software. As shownin FIG. 1, in this embodiment, external memory modules 127, 128 and 129are coupled to digital baseband circuit 107 through external memoryinterface (EMIF) 126. External memory modules 127, 128 and 129 are usedin this embodiment to provide data memory, program memory andpositioning data memory for CPU subsystem 131. (Positioning data memorystores samples of a positioning signal and data used in positioningsignal detection.) In other embodiments, data memory, program memory andpositioning data memory can be provided by built-in memory modules indigital baseband circuit 107. Alternatively, positioning data memory 129and data memory 128 can reside in the same physical memory module. Asshown in FIG. 1, CPU subsystem 131 communicates with RXIF 111, TXIF 112,clock generation circuit 113, timing generation circuit 114 andcommunication and I/O ports 115 over a direct memory access (DMA) andtraffic control circuit 116.

[0026] As shown in FIG. 1, CPU subsystem 131 includes CPU 130, randomaccess memory (RAM) 123, read-only memory (ROM) 124 and cache memory131. CPU 130 communicates with RAM 123, ROM 124 and cache memory 131over a processor bus. Bridge 122 allows data to flow between theprocessor bus and DMA and traffic control circuit 116. As known to thoseskilled in the art, software executed by CPU 130 can be stored in anon-volatile fashion in ROM 124 and also in external program memory 128.RAM 123 and cache memory 131 provide the memory needs of CPU 130 duringits operation.

[0027] In the embodiment shown in FIG. 1, digital signal processor (DSP)subsystem 132 is provided in digital baseband circuit 107. DSP subsystem132 can be used to execute computationally intensive tasks, such asencoding and decoding voice samples, and executing the tasks of thephysical layer in communication with a station. DSP subsystem 130 can beimplemented, for example, substantially the same organization as CPUsubsystem 131, and provided similar access to external memory modules127, 128 and 129 through EMIF 126.

[0028] The positioning signal receiver software, which detects GPSsignals from multiple GPS satellites to determine the location of mobilecommunication device 100, may be run on either CPU subsystem 131 or DSPsubsystem 132. In this embodiment, the digitized samples of the receivedGPS signal are stored in memory. The stored GPS signal samples are thenlater retrieved and processed to search for the GPS satellite, the codephase and frequency shift (“Doppler”) that would provide the receivedsignal. In one embodiment, positioning receiver software searches for apeak in the modulus of a complex correlation integral under hypothesizedcode phase, Doppler and integration time values. One example of suchpositioning receiver software is disclosed in copending patentapplication (“Copending Application”), Ser. No. _______ , entitled“Method for Optimal Search Scheduling in Satellite Acquisition” by J.Stone et al., filed on or about the same day as the present application,Attorney Docket number M-12558 US, assigned to Enuvis, Inc., which isalso the Assignee of the present application. The disclosure of theCopending Application is hereby incorporated by reference in itsentirety.

[0029] If the mobile communication device, in response to communicationwith a base station, adjusts the frequency of shared local oscillator108 while the GPS signal is being captured, a discontinuity appears as astep shift in carrier frequency in the digitized GPS signal. The presentinvention compensates for this carrier frequency shift in the complexcorrelation integral.

[0030]FIG. 2 shows a flow chart of a method that compensates for thefrequency adjustment in shared local oscillator 108, in accordance withthe present invention. As shown in FIG. 2, at step 201, the dataprocessing portion of the positioning signal receiver software (PRXPF)receives a request for the user's position (e.g., the user selecting a“get position” command from a menu), or from an external source (e.g., aprotocol request in a message relayed from the base station). At step202, PRXPF retrieves aiding data (e.g., GPS ephemeris, approximatelocation, and time) from local storage or memory, or from an externalsource (e.g., by protocol messages to a server sent over a radiocommunication link to a base station). At step 203, the control portionof the positioning signal receiver software (PRXCF) initializespositioning signal receiver 103 to begin storing samples between time t₀to time t₂. Time may be measured, for example, in mobile communicationdevice 100's local time base or relative to a timing event in thecommunication link between mobile communication device 100 and a basestation (e.g., a frame boundary in the radio communication interface toa base station). At time t₀, the contribution to the residual carrierfrequency due to shared local oscillator 108 should ideally be zero.

[0031] Suppose at time t₁ (t₀<t₁<t₂), mobile communication device 100adjusts the frequency of shared local oscillator 108 by an amount suchthat the residual carrier frequency in the positioning signal sampleschanges by Δf₁. Thus, at step 204, a record is made in mobilecommunication device 100, noting the time of the frequency adjustmentand the amount of the frequency adjustment. At time t₂ (step 205), PRXCFturns off positioning signal receiver 103. At step 206, using theknowledge of the time and amount of the frequency adjustment, PRXPFperforms the complex correlation integration using different hypothesesof a frequency offset due to shared local oscillator 108, according towhether the integration time limits are with the [t₀, t₁] interval or[t₂, t₃] interval. That is:

f _(VCO)(t)=0, for t ₀ <t<t ₁

f _(VCO)(t)=Δf ₁, for t ₀ <t<t ₁

[0032] At step 207, using the compensated integration of step 206, PRXPFexecutes the remainder of PRXPF to obtain the pseudo-range and,consequently, the position of mobile communication device 100.

[0033] For any reason, if either the frequency adjustment time t₁ or theamount of frequency adjustment cannot be ascertained, the frequencyadjustment time t₁ or the amount of frequency shift due to shared localoscillator 108 (i.e., Δf₁) are considered additional search parameters.For example, if the frequency adjustment time t₁ is known, but thefrequency adjustment amount is not known, multiple hypothetical valuesfor Δf₁ can be used to search for Δf₁. Alternatively, if the frequencyadjustment time t₁ is not known, but the frequency shift due to sharedlocal oscillator 108 is known, multiple hypothetical values for t₁ canbe used to search for time t₁. Of course, if neither the frequencyadjustment time t₁ nor the frequency shift due to shared localoscillator 108 is known, both the time and frequency parameter spaceshave to be searched. In any case, the frequency adjustment software inthe mobile communication device notifies the PRXPF that such anadjustment has occurred, and provides as much information related to theadjustment as is available.

[0034]FIG. 3 shows a block diagram of mobile communication device 300,to which is a method of the present invention is also applicable. Unlikemobile communication device 100 of FIG. 1, mobile communication device300 uses a CPU or DSP 151 which resides outside of digital basebandcircuit 107. Other than where the positioning signal receiver softwareand data reside and execute, the operation of mobile communicationdevice 300 and mobile communication device 100 with respect to locationdetermination and compensation for frequency adjustment in shared localoscillator 108 are substantially identical.

[0035] According to another aspect of the present invention, thefrequency of a local oscillator (e.g., shared local oscillator 108 orthe higher frequency output signal of a phase-locked loop) can bedetermined using the oscillator of a base station. The present inventionuses a timing signal of known duration, or having events of knownrecurring frequency, as a reference or “stop watch” signal to measurethe actual local oscillator frequency. For example, in a CDMA network, a“short code” of 26⅔ millisecond duration is broadcast on a pilotchannel. The frequency of the short code rollover at 37.5 Hz can be usedfor synchronization. Alternatively, a “long code” broadcast on a CDMAnetwork can also be used to synchronize a 10 MHz source. Each code has astarting point and an ending point indicated by a predetermined pattern.Similarly, in a GSM network, the Broadcast Control Channel (BCCH)transmitted by the base station includes a Synchronization Channel (SCH)having counts indicating the positions of the current frame within amulti-frame, super-frame and hyper-frame structures. The multi-frame,super-frame and hyper-frame structures have respective durations of0.235 seconds, 6.12 seconds and approximately 3 hours and 29 minutes.Thus, in a GSM network, the starting points of successive mult-framescan be used as fixed time intervals. Other intervals inherent in the GSMair-interface frame structure can also be used as fixed time intervals.In addition, a counter is provided in the hardware that is clocked by aclock signal generated from shared local oscillator 108. In oneembodiment, a nominally 200 MHz signal from a PLL in positioning signalreceiver 103 is used to clock the counter.

[0036]FIG. 4 illustrates method 400 for measuring the operatingfrequency of shared local oscillator 103, in accordance with the presentinvention. As shown in FIG. 4, step 401 detects a starting point in theselected reference signal from the base station. At step 402, when thestarting point in the reference signal is detected, the counter is resetto enable count, incrementing one for each cycle of its input clocksignal. In one embodiment, detecting the starting point and starting thecounter can be accomplished by software running in CPU subsystem 130. Inother embodiments, these functions can be carried out in hardware. Atstep 403, when the ending point of the reference signal is detected, thecounting is disabled. At that time, the count in the counter representsthe number of clock cycles elapsed between the starting and ending pointof the referenced signal (i.e., the fixed time interval). The frequencyof shared local oscillator 108 is thus simply this fixed time intervaldivided by the count in the counter. An adjustment to the count may bedesirable to account for the latencies in signal detection and thecounter operations for higher accuracy.

[0037] In one embodiment, shared local oscillator 108 can operatebetween 10-25 MHz. A PLL in positioning signal receiver 103 multipliesthe oscillator frequency to 200 MHz. Theoretically, the uncertainty inthis 200 MHz signal under method 400 in that embodiment is estimated tobe 10 Hz. However, nondeterministic latencies (e.g., due to the tasks inCPU subsystem 130) brings the uncertainty up to about 100 Hz.

[0038] Using the method of the present invention, the operatingfrequency of shared local oscillator 108 can be determined to theaccuracy of the oscillator of the base station oscillator, withoutincurring the expense or inconvenient bulkiness of the more costlyhigher precision oscillator typically found in base stations. Byremoving the uncertainty in oscillator frequency, the Doppler range overwhich the positioning signal receiver software searches can be furtherlimited.

[0039] The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modifications within the scope of the presentinvention are possible. For example, the detailed description abovedescribes a system in which the positioning signal receiver stores thesampled received signal and later retrieves the stored data forprocessing. Another embodiment which processes the sampled data as theyare sampled is within the scope of the present invention. The presentinvention is set forth in the following claims.

We claim:
 1. In a mobile communication device, a method for compensatingfor a frequency adjustment in an oscillator shared between acommunication circuit and a positioning signal receiver, comprising: ata first point in time, beginning receiving into a positioning signalreceived from the positioning signal receiver; at a second time point,adjusting a frequency of the oscillator; recording a frequencyadjustment of the oscillator; at a third time point, completingreceiving the positioning signal into the positioning signal receiver;and processing the positioning signal, taking into consideration thefrequency adjustment.
 2. A method as in claim 1, wherein the recordingcomprises recording the second time point as the time at which thefrequency adjustment of the oscillator is made.
 3. A method as in claim2, wherein the processing hypothesizes a frequency shift in the receivedpositioning signal between the second time and the third time.
 4. Amethod as in claim 1, wherein the recording comprises recording anamount by which the frequency of the oscillator is adjusted.
 5. A methodas in claim 4, wherein the processing hypothesizes a time point at whichthe frequency adjustment of the oscillator is made.
 6. A method as inclaim 1, wherein the processing comprises integrating a correlationfunction.
 7. A method as in claim 6, wherein the processing comprisessearching for a code phase for which the correlation function has asignificant value.
 8. A method as in claim 6, wherein the processingcomprises searching for a Doppler frequency at which the correlationfunction has a significant value.
 9. A method as in claim 1, furthercomprising retrieving aiding data for processing the positioning signal.10. A method as in claim 1, wherein the positioning signal comprises asignal from a global positioning system (GPS) satellite.
 11. A method asin claim 1, wherein the communication circuit initiates the frequencyadjustment of the oscillator.
 12. A method as in claim 11, wherein thecommunication circuit comprises a receiver and transmitter for cellulartelephone communication.
 13. A method as in claim 12, wherein thefrequency adjustment of the oscillator adjusts the offset from a nominalfrequency in a communication signal transmitted from a base station. 14.In a communication device, a method for determining an operatingfrequency of an oscillator based on a reference signal from a reliabletime base, comprising: detecting a beginning time point of the referencesignal received by the communication device; upon detection of thebeginning time point of the reference signal, enabling a counter tocount in accordance with a clock signal derived from the oscillator;detecting an ending time point of the reference signal received by thecommunication device; upon detecting the ending time point of thereference signal, disabling the counter to stop the counter from furthercounting; and determining the frequency of the oscillator based on thecount in the counter and an expected time that elapsed between thebeginning time point and the ending time point.
 15. A method as in claim14, wherein the beginning time point and the ending time point representa known duration in time.
 16. A method as in claim 14, wherein thebeginning time point and the ending time point represent arrivals ofrecurring events in the reference signal, the recurring events recurs ata fixed frequency.
 17. A method as in claim 14, further comprisingadjusting for processing times in the communication device for detectingthe beginning time point and the ending time point.
 18. A method as inclaim 1, wherein the recording comprises recording an indication that afrequency adjustment of the oscillator is made.
 19. A method as in claim18, wherein the processing hypothesizes a frequency shift and time pointof the frequency shift in the received positioning signal between thesecond time point and the third time point.
 20. A method as in claim 14,wherein the frequency of the oscillator thus determined is provided to apositioning signal receiver.
 21. A method as in claim 20, wherein thepositioning signal receiver receives global positioning system (GPS)signals.
 22. A mobile communication device, comprising: a communicationcircuit; a positioning signal receiver; an oscillator shared between acommunication circuit and a positioning signal receiver providing afrequency that is adjustable from the communication circuit; and acentral processing unit, wherein the central processing unit records afrequency adjustment of the oscillator, and processes the positioningsignal, taking into consideration the frequency adjustment.
 23. A mobilecommunication device as in claim 22, wherein the central processing unitrecords the time point at which the frequency adjustment of theoscillator is made.
 24. A mobile communication device as in claim 23,the central processing unit hypothesizes a frequency shift in a portionof the received positioning signal received into the positioning signalreceiver after the frequency adjustment.
 25. A mobile communicationdevice as in claim 22, wherein the central processing unit records anamount by which the frequency of the oscillator is adjusted.
 26. Amobile communication device as in claim 22, wherein the centralprocessing units hypothesizes a time point at which the frequencyadjustment of the oscillator is made.
 27. A mobile communication deviceas in claim 22, wherein the central processing unit integrates acorrelation function.
 28. A mobile communication device as in claim 27,wherein the central processing unit searches for a code phase for whichthe correlation function has a significant value.
 29. A mobilecommunication device as in claim 27, wherein the central processing unitsearches for a Doppler frequency at which the correlation function has asignificant value.
 30. A mobile communication device as in claim 27,wherein the central processing unit retrieves aiding data for processingthe positioning signal.
 31. A mobile communication device as in claim22, wherein the positioning signal comprises a signal from a globalpositioning system (GPS) satellite.
 32. A mobile communication device asin claim 22, wherein the communication circuit comprises a receiver andtransmitter for cellular telephone communication.
 33. A mobilecommunication device as in claim 22, wherein the frequency adjustment ofthe oscillator adjusts the offset from a nominal frequency in acommunication signal transmitted from a base station and received by thecommunication circuit.
 34. An oscillator frequency determining apparatusin a communication device, comprising: an oscillator providing aperiodic output signal; a receiver receiving a reference signal from areliable time base; a detector detecting a beginning time point and anending time point of the reference signal received by the communicationdevice; a counter that begins counting the number of periods in theoutput signal of the oscillator in response to the detector detectingthe beginning time point and stops counter in response to the detectordetecting the ending time point of the reference signal; and arithmeticunit for determining the frequency of the oscillator based on the countin the counter and an expected time that elapsed between the beginningtime point and the ending time point.
 35. An apparatus as in claim 34,wherein the beginning time point and the ending time point represent aknown duration in time.
 36. An apparatus as in claim 34, wherein thebeginning time point and the ending time point represent arrivals ofrecurring events in the reference signal, the recurring events recurs ata fixed frequency.
 37. An apparatus as in claim 34, wherein thearithmetic unit further adjusts for processing times in thecommunication device for detecting the beginning time point and theending time point.
 38. A mobile communication device as in claim 22,wherein the central processing unit records an indication that afrequency adjustment of the oscillator is made.
 39. A mobilecommunication device as in claim 38, wherein the central processing unithypothesizes a frequency shift and time point of the frequency shift inthe received positioning signal between the second time point and thethird time point.
 40. An apparatus as in claim 34, wherein the frequencyof the oscillator thus determined is provided to a positioning signalreceiver.
 41. An apparatus as in claim 40, wherein the positioningsignal receiver receives global positioning system (GPS) signals.